[38 coupe]

The bump is where you line up the initial ignition timing, the details of 2 or 4 degrees BTDC is academic.
There is not a mechanical advance in the factory 8BA style distributor. The factory distributor works very well if you have a stock or near stock camshaft, a single stock carburetor, and you hook up the vacuum advance line to the correct port on the carburetor. JWL did extensive testing and found a stock 8BA style distributor provided optimal timing for maximum power with a stock engine.